This application is based upon and claims the benefit of Japanese Patent Application No. 2001-140556 filed on May 10, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor pressure sensor, and specifically to a semiconductor pressure sensor that can detect minute pressure.
2. Background of the Invention
Generally, a semiconductor pressure sensor includes a semiconductor substrate, a diaphragm and diffusion gauge resistors. The diaphragm is formed in the main surface of the substrate. The diffusion gauge resistors are formed by ion implantation and diffusion. Then, a detection signal corresponding to pressure applied to the diaphragm can be generated based on a resistance value change of the diffusion gauge resistors.
Furthermore, in a thick outer portion of the diaphragm, metal wiring is formed on the main surface of the substrate for electrically connecting the diffusion gauge resistors to a component outside of the substrate and for outputting the pressure signal through the diffusion gauge resistors.
Such a semiconductor pressure sensor is typically made with a substrate having a (110) surface as the main surface (hereinafter referred to as a 110 type substrate), because the influence of thermal stress on the 110 type substrate is smaller than on a substrate having a (100) surface as the main surface (hereinafter referred to a 100 type substrate).
Further, the following limitations are associated with a semiconductor pressure sensor using the 110 type substrate to improve its sensitivity.
Namely, the metal wiring formed on the thick portion is made by depositing Al (aluminum) or the like, but creep stress generated in the metal wiring is supplied to the diaphragm. Therefore, the stress changes the sensor output. For example, the sensor is subjected to high temperature in a bonding process for adhering the sensor to a case. However, when the sensor is returned to room temperature after application of heat, the thermal stress of the metal wiring is moderated over several hundred hours. Then, stress generated based on the creep stress in the metal wiring is supplied the diaphragm, and the sensor output fluctuates so that the diaphragm is deformed thereby. Therefore, the sensor output includes an error even if trimming is done to the sensor to adjust the output thereof after the bonding process.
As for an intermediate sensor detecting range (e.g., 100 kPa), the problem of the output change by creep stress of the metal wiring is not conspicuous because an offset of the sensor output is small.
However, it has been proven that, for example, an offset of the sensor output caused by creep stress of the metal wiring may be approximately 1 FS %.
It is therefore an object of the present invention to provide a semiconductor pressure sensor that is capable of obviating the above problem.
It is another object of the present invention to provide a semiconductor pressure sensor that is capable of repressing output change due to creep stress of the metal wiring segments.
If a thin diaphragm having a large area can be adapted, it will be possible to realize a semiconductor pressure sensor that can detect minute pressure. A semiconductor pressure sensor having a ratio S/d is larger than 100, where an area of the diaphragm 12 is Sxcexcm2 and a thickness thereof is dxcexcm. In this constitution, minute pressures as small as 10 kPa can be detected.
As shown in FIG. 3, there are two perpendicular crystalline axes, i.e.,  less than 110 greater than  and  less than 100 greater than  crystalline axes, that run at right angles relative to one another in the (110) surface that is the main surface of the diaphragm.
Further, the stress sensitivity of the direction of the  less than 110 greater than  crystalline axis is much larger than that of the direction of the  less than 100 greater than  crystalline axis. For example, the former is fifty times more sensitive than the latter. That is, a piezo-resistance coefficient of the direction of the  less than 110 greater than  crystalline axis is larger than that of the direction of the  less than 100 greater than  crystalline axis. Therefore, the  less than 110 greater than  crystalline direction is used for pressure detection on the (110) surface.
As the direction of the  less than 110 greater than  crystalline axis exists only in the (110) surface, diffusion gauge resistors have to be arranged as shown in FIG. 3 if higher output is gained using the direction of the  less than 110 greater than  crystalline with high sensitivity. Namely, center gauges are arranged close to the center of the diaphragm, and side gauges are arranged at peripheral position of the diaphragm in comparison with the center gauges. Further, a bridge circuit includes by the four diffusion gauge resistors by which stress generated in the direction of the  less than 110 greater than  crystalline is detected.
Accordingly, the present invention is created under the above presupposition to decrease creep stress that acts in the direction of the  less than 100 greater than  crystalline axis.
According to the present invention, metal wiring segments arranged peripherally on the diaphragm are formed on a main surface of a thick portion of a semiconductor substrate. A ratio S/d is larger than 100, where an area of the diaphragm is Sxcexcm2and a thickness thereof is dxcexcm. Further, the total area of the metal wiring segments arranged on first sides is larger than the total area of the metal wiring segments arranged on second sides, where the first sides indicate the sides in parallel with the  less than 110 greater than  crystalline axis and the second sides indicate the sides in parallel with the  less than 100 greater than  crystalline axis.
When the metal wiring segments are primarily arranged on the side of the first sides in parallel with the  less than 110 greater than  crystalline axis, the creep stress in the metal wiring segments, which acts on the diaphragm, mainly acts in the direction of the  less than 100 greater than  crystalline axis. Therefore, the creep stress that acts the direction of the  less than 110 greater than  crystalline axis relatively decreases.
Therefore, in the semiconductor pressure sensor having the diaphragm that is constructed with the 110 type substrate, a change of the sensor output based on the creep stress in metal wiring segments can be repressed when the sensor that can detect minute pressure is employed as the semiconductor pressure sensor.